Pumps static suction head

Static Suction Head The static suction head / is the vertical distance measured from the free surface of the hquid source to the pump centerhne plus the absolute pressure at the liquid surface. 

The total head on the pumps in Fig. 6.13d, c, and /is computed in the same way as described above, except that the total static head is less because the pump has a static suction head—that is, the elevation of the liquid on the suction side reduces the total distance through which the pump must discharge liquid thus the total static head is less. The static suction head is subtracted from the static discharge head to determine the total static head on the pump. 

Compute the net positive suction head on the pump. The net positive suction head hn on a pump when the liquid supply is above the pump inlet equals pressure on liquid surface + static suction head-friction-head loss in suction piping and pump inlet-vapor pressure of the liquid, all expressed in feet absolute of liquid handled. When the liquid supply is below the pump centerline—i.e., there is a static suction lift—the vertical distance of the lift is subtracted from the pressure on the liquid surface instead of added as in the preceding relation. 

The terms suction and discharge in the context of heads refer to portions of the system before and after the pumping station, respectively. Static suction lift h( is the vertical distance from the elevation of the inflow liquid level below the pump inlet to the elevation of the pump centerline or eye of the impeller. A lift is a negative head. Static suction head h, is the vertical distance from the elevation of the inflow liquid level above the pump inlet to the elevation of the pump centerline. Static discharge head h is the vertical distance from the centerline elevation of the pump... 

Static suction head—The vertical distance from the elevation of the inflow liquid level above the pump centerline to the centerline of the pump. 

STATIC SUCTION HEAD - The positive vertical height in feet from the pump centerline to the top of the level of the liquid source. 

It is difficult to determine exactly the areas of localized pressure reductions inside the pump, although much research has been focused on this field. It is easy, however, to measure the total fluid pressure (static plus dynamic) at some convenient point, such as pump inlet flange, and adjust it in reference to the pump centerline location. By testing, it is possible to determine the point when the pump loses performance appreciably, such as 3% head drop, and to define the NPSH at that point, which is referred to as a required NPSH (NPSHR). The available NPSH (NPSHA) indicates how much suction head... 

Net Positive Suetion Head (NPSH) The net static liquid head that must be provided on the suction side of the pump to prevent cavitation. 

The total suction lift is defined as above except the level of the liquid is below the centerline of the pump or the head is below atmospheric pressure. Its sign is negative. Total Suction Lift (TSL) = static lift plus friction head losses. 

Qni = Flead at best efficiency point on pump curve, ft = Viscous liquid capacitv, GPM Q = Water capacity, GPM S =. Suction static head, ft, or height of liquid (static) above ( + ) or below ( —) the center line of the pump on suction side, ft, or,... 

Hs0 = Head at no flow, or shutoff, ft I4ms = Head of viscous fluid, ft Hw = Water equivalent head, ft hd = Discharge head on a pump, ft of fluid hs = Suction head (or suction lift) on a pump, ft of fluid hSL, hDL = Friction losses in pipe and fittings , subscript SL for suction line and DL for discharge line, ft of fluid hv = Velocity head, ft of fluid L = S = Static head, suction side, ft (Figure 3-38)... 

Centrifugal pumps, 181 Discharge systems, 187 Example calculation, 186 Flow friction losses, 185. 186 Friction losses, pipe, see Chapter 2 Friction, 188 Pressure head, 184—186 Static head, 184-186 Suction head, 184, 185 Suction lift, 184, 185 Suction systems, 186 Hvdroclones, 265—267 Application system, 267 Ignition, flammable mixtures, 493 Impellers, centrifugal, reducing diameter, 203 Impellers,... 

It is clear from equation 4.2 that the suction head hs can fall to a very low value, for example when the suction frictional head loss is high and the static head zs is low. If the absolute pressure in the liquid at the suction flange falls to, or below, the absolute vapour pressure Pv of the liquid, bubbles of vapour will be formed at the pump inlet. Worse still, even if the pressure at the suction flange is slightly higher than the vapour pressure, cavitation—the formation and subsequent collapse of vapour bubbles— will occur within the body of the pump because the pressure in the pump falls further as the liquid is accelerated. 

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

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